Choice of Cameras for Beginner @ Photometry

I am getting my equipment package together to start doing variable star photometry. My long term goals are to be able to contribute meaningful data in VS photometry and to work up to exoplanet light curves as well. I have two used cameras I am looking at for this purpose. Will be using a C925HD OTA with a focal length of 2350mm operating at f/10 mounted on an Atlas EQ-G mount.

My mentor, Tim Crawford, has suggested I reach out to this forum for advice. I would appreciate help from the group as to which one would be my best choice or if there are any other factors I should be considering. My choice so far is between a ST-8XE NABG w/CFW8, Custom Scientific LRGB and an Ha filters. With this I will have 9 micron pixels and water assisted cooling (which is important for my warm SoCal climate.) Or, a ST-2000XM ABG w/CFW10 and no filters (I already have an Astrodon V filter). With this I will be working with 7.4 micron pixels and water assisted cooling as well. The cameras are about the same age and priced close enough that it isn't a factor.
Thanks for your help!
John Downing
DJCA

Get the camera with the larger pixels, the ST8XE. You'll probably want to use a focal reducer as well. If you can get effective focal ratio down to ~F7 your image scale would be 1.13 arcseconds per pixel. This is good sampling if you'll lucky enough to have 2-3 arcsecond FWHM seeing.

If your seeing is the more typical 4+ arcseconds consider looking for an ST9e or ST9Xe with 20 micron pixels. You wouldn't need to use a focal reducer and your image scale would be 1.75 arcseconds per pixel, much better suited to 4-5 arcsecond seeing. The larger pixels and deeper full well depth make this a very good camera for photometry. All the ST9's are NABG so no need to worry about that.

Ask imagers in your area what their typical seeing is. Tell them to be brutally honest. Some people hate to admit just how bad their typical seeing really is.

A local amateur and AAVSO observer tells me our seeing is typically 2-3, hopefully that will prove to be true! The estimate was done by Tim but I don't know if the data was typical. As we are only about 8 miles from Palomar I am willing to plan on <4 as a rule. I will have a Focal reducer available if it is absolutely necessary.

Each of these cameras would probably do a good job, but for photometry, the ST-8 is probably the better choice. The larger pixels and NABG means the pixels will store more electrons. In the ABG cameras like the ST-2000 part of the pixel space is used by the drain so not only does it not store as many electrons, but less surface area receives light so its QE or quantum efficiency is lower.

You can enter your camera and scope choices in the CCD calculators on line. An easy one to use is at:

You'll see that with your scope the ST-8 gives a .79 arc"/pixel and a 20x13' FOV. The ST-2000 is .65 arc"/pixel and 17'x13' FOV. These numbers are really a wash, the differences are pretty small. If your seeing is in the 3 arc" range both will have over 3 pixels of width under a star image, which is a good sampling.

These fields of view are workable but on the small side. You can increase the FOV by using a focal reducer. The reducer adds optical elements thus reducing the throughput and changes the pixel resolution. The .79 arc"/pixel of the ST-8 becomes 1.25 arc"/pixel so the sampling is reduced. With the same seeing, there are now just under 3 pixels under a star image. For standard pretty picture imaging, the smaller pixels would work better with the shorter focal length.

Water systems are another toss up. Yes, they do improve cooling, some, at the cost of inconvenience. All the tubing, routing it and pump is really a hassle, especially if you're working portable. My experience was that SBIG camera only seem to get about 5 degrees adding water.

So as you can tell, there is no 'perfect' combination, they've all got compromises. The ST-8 on this scope sounds like a good combination, but don't be surprised if you end up altering it some as you gain some experience!

John:
I would strongly recommend the ST-8 over the ST-2000. I've been using the ST2000 for several years now and am very happy with it. However, it has a full well depth of about 1/3 that of the ST-8 which means that it can't achieve as good a signal-noise ratio. Because of the ABG it needs to be very carefully tested for linearity etc. The 7.4micron pixels are smaller than you need. The only potential upside to the ST-2000 over the ST-8 is that its quantum efficiency doesn't drop off in the blue as badly. But the quantum efficiency is lower in general so that doesn't really help.

Back when I was buying, the deciding factor was the ST-2000's increased field of view over any other camera of comparable price (and it included the filter wheel at that price.) The ST-8 was nearly twice the price.

Nice thread! I'm also looking to get a new camera to increase my SNR. A ST-8XE type camera looks good except for the small FOV it offers over more modern cameras. I seem to be getting good results with my ABG STL-11K, as long as the ADU for my reference, check stars, or object doesn't go above 20K; after this, my camera has a non-linear response. I'm seeing a "typical" linear response on ST-8 type cameras out to about 40K-45K or over twice the range my camera's linearity. An "ideal" replacement for me would be a STL-6303 camera with about 80% the area of my STL-11K. This camera has 4X the area of the venerable ST-8 and a larger guide chip and is NABG. Alas it far harder to find on the used market than an ST-8 and the newer SBIG STT 6303 is over $8K with filters.

If we're strictly talking photometry (and bang for the buck with a used camera) with 3 arcsec seeing and a C925, I think you're better off with the ST8XE or, better yet, the ST9XE or even the ST9E. Since we are now blessed with APASS it is possible to get a good selection of high quality comparison stars in almost any 10-15 arcminute FOV. For me larger pixels and deeper well depth trump the larger FOV.

If only photometry were the only factor.....there are two are other important constaints:

1.) Having a bright guide star centered in the guide chip thru a rather opaque B filter

2.) Having the coverage to get all those "juicy" comp stars while composing the frame to get aformentioned guide star to lock on for a 2-7 minute exposure.

I bring this up not to debate, but to illuminate my struggles with getting a guide star in the "puny" ST-7E I've occasionally used with a 10" reflector at 1300mm focal length. Since I do automated runs and dont have a rotator, I need a field of view with at least 5 comp stars and reference star, as well as the variable, to reduce in MaximDL. This is a daunting challenge, as some of the F-scale (18' wide) charts have less than 4 reference stars around the object. Using the STL-11K on a 2,500mm reflector is much easier to do this than using a smaller detector with a scope at 1/2 the focal length; abiet the ST-8 has 4x the area of the ST-7.

Having a large FOV helps bag all these scattered comps plus the variable. Probably the best all around for "deep well" capacity and large FOV coverage was the 1kX1k SBIG 1001 chip with 24 micron pixels! Alas, this is even harder to find used than all the others cameras mentioned.

"...on the first rung of the photomety ladder so to speak. I purchased an NABG ST7 ME and a Johnsons V filter (thanks Hubert, HHU) and are using it with an Orion ED80, 39'x26' fov and ~3.0" per pixel."

Doug,

I noticed your post this morning to John's thread on the CCD Forum. If your image scale is ~ 3 arcsec/pixel you are potentially going to be servery under sampled (unless your seeing runs around 6 arcsec, in which case you are in good shape) and from my experience, if so, this often results in spurious data.

For good sampling we want to spread the light out over a minimum of two pixels. See section 2.2 of the AAVSO CCD manual.

For several years I used a ST7 on an 8" SCT with a FOV of 8'x12' (if I remember right). The Fov was very workable. I just had to choose targets targets that had close comp stars. I still have that camera and have considered putting it on a short tube 80 for a poor man's BSM.

In your case, I wouldn't worry too much about being undersampled. At 3 arcsec/pixel on most nights you are still going to span more than one pixel. You can always try defocusing a tiny bit and see if you get better results. You can also image in the 2x2 mode which would completely remove any issue over being undersampled.

Observe, enjoy and be sure and submit your observations to the AAVSO when you feel you are ready.

Obviously binning won't help in this situation. It only makes things worse. One of those 'Oh yesh's" that I threw in without thinking about it enough.

Defocusing slightly can help.

If you were buying a camera for optimal results you would want to purchase a camera that matches your telescope and seeing. The important word in that sentence is optimal. But you already have your equipment and probably want to "cook with what's in the kitchen". Go fo it.

Thanks for the responses! I very much appreciate your sharing your experiences with me and giving your advice. We seem to all agree that for strictly photometry purposes the ST8 is the best choice for my current needs and budget. I do understand the smaller fov will be a challenge finding reference stars but have practiced with the plotter and can make it work within my fov for many targets. As far as guiding is concerned I forgot to mention I will be using a guide scope and auto-guider. I tried years ago to use the internal guide chip on an ST7 for color imaging and it was difficult.

It seems to me it would be beneficial to have a data base of what CCD cameras are being used for photometry. Perhaps indicating such information as focal length, typical seeing, area of study, level of experience, etc. Would it not be of help to beginners like me and also for those who are looking to upgrade, change telescopes, and/or enter into a new areas of work?

My initial worry was getting a good fov. I've now decided, with some priceless knowledge from Tim (CTX)to use my 6 inch refractor instead, under sampling won't be an issue at 1.55"/pixel and my fov will be 19.73' x 13.15'.